Conventionally, thin-film semiconductor devices referred to as thin-film transistors (TFTs) have been used for active matrix display devices such as liquid crystal display devices and solid-state imaging devices such as digital cameras. In display devices, the TFTs are used as switching elements which select pixels, driving transistors which drive pixels, drivers outside of panels, or the like.
For instance, unlike a voltage-driven liquid crystal display, an organic EL display including organic electroluminescence (EL) elements using EL of an organic material is a current-driven display device. Thus, there is an urgent need for development of a high-performance thin-film transistor. A thin-film transistor includes, above a substrate, a gate electrode, a semiconductor layer (a channel layer), a source electrode, and a drain electrode. The channel layer generally includes a silicon thin film.
Moreover, it has been demanded that a screen size of a display device be increased and costs of the display device be reduced. In general, a bottom-gate thin-film transistor whose gate electrode is formed closer to a substrate than to a channel layer is used as a thin-film transistor which easily allows cost reduction.
Bottom-gate thin-film transistors are broadly categorized into two types, that is, a channel etching thin-film transistor whose channel layer is etched, and a channel protecting (etching stopper) thin-film transistor whose channel layer is protected from etching. Of these, the channel protecting thin-film transistor is capable of preventing damage to the channel layer by the etching, and of reducing an increase in characteristic variation in the surface of the substrate. In addition, the channel protecting thin-film transistor is capable of reducing the thickness of the channel layer and increasing ON characteristics by reducing parasitic resistance component, which is advantageous for high definition.
For this reason, the channel protecting thin-film transistor is suitable for a driving transistor in a current-driven organic EL display device using organic EL elements, for example.
Moreover, as stated above, because the organic EL element is the current-driven element, a thin-film transistor driving the organic EL element is preferably a polycrystalline silicon TFT including polycrystalline silicon excelling in a current drive capability. Consequently, it is possible to obtain a high current drive capability in a stacked TFT by forming a semiconductor layer, a source, and a drain with polycrystalline silicon. Moreover, especially in a p-type TFT, when a non-crystalline silicon film is used for a source and a drain, a resistance value becomes high significantly. For this reason, when the p-type TFT and a CMOS-type TFT using the p-type TFT are formed, the p-type TFT is preferably the polycrystalline silicon TFT.
As such a technique, a display device has been developed in which CMOS-type driving circuits advantageous for low power consumption are evenly formed in a plane using stacked TFTs including polycrystalline semiconductors (e.g., see Patent Literature (PTL) 1).